A conventional power transforming device protection and control system to which a process bus is applied includes voltage/current detection means called a merging unit (MU) or a sensor unit, and protection and control means called an IED (intelligent electronic device). These merging unit and IED are interconnected via a communication network called a process bus. Furthermore, the IED is coupled to a higher control computer terminal via a communication network called a station bus.
In recent years, it has been proposed to duplicate these process bus and station bus (more generally, to make them redundant) to improve reliability (see the IEC 62439-3 standard specification (NPD 1), for example). Japanese Patent Laying-Open No. 2002-315233 (PTD 1) discloses duplication of transmission paths.